A light detector based on ‘phototube’ device, such as a hybrid photon detector (HPD) sensor, may work at high gains (for example—gains that range between 100 and 200,000).
These high gains are required in order to detect very weak signals (even single photons)—such as weak signals from the bottom of high aspect ratio holes in wafer scanning applications.
When scanning a wafer with high contrast, certain patterns of the wafer (usually highly reflective patterns) reflect a light beam that includes many photons for long durations thereby resulting in long durations of high anode currents. The long duration may be regarded as about one micro-second and high anode currents have a magnitude which may even reach a few tens of milli-Amperes.
The long duration high anode current may cause the HPD sensor to be damaged or otherwise heated. The heating of the HPD sensor may result in gain changes that are hard to measure or predict.
There is a need to provide a light detector of high gain that will withstand long durations of strong light beams.